An improved circuit for enhancing the print quality of a laser raster output scanner (ROS), and more specifically, architecture for reducing the amount of comparator circuitry needed to compare a large number of templates, each having a large number of bits, to an image area surrounding the pixel to be corrected, to produce a video correction signal.
In a typical laser driven raster output scanner, output pixels are either black or white. A problem with lower cost printers is that in an image having a reduced number of pixels per inch, diagonal lines appear to be jagged as a result of relatively large pixels being added to or subtracted from each succeeding raster. This results in artifacts in the form of steps in what should appear as a straight line. This problem is clearly shown in U.S. Pat. No. 5,005,139 which is incorporated by reference herein. In the left center of FIG. 3 of that patent there is shown the result of attempting to print a diagonal line with vertical rasters, and the jagged appearance of the result.
One common solution to this problem is to move to a higher resolution. Therefore, while the artifact is noticeable at 200 spots per inch, it is considerably less noticeable at 600 or more spots per inch.
A lower cost option is to keep the same resolution but to intervene just before the laser engine to vary the shape and/or size of the spots. For example, if the top one or two pixels within the circle 71 in FIG. 3 of the incorporated patent could be shifted slightly to the left, and the bottom one or two could be shifted slightly to the right, the appearance would be considerably improved.
This can be accomplished by printing dots at some fraction of their normal intensity to either the right or left of the main dots, and/or by printing the main dots at a fraction of their full intensity, as shown in the center 75 of FIG. 3. Because close laser dots tend to clump together, the effect is to shift the position of the dot to the right or left, thereby improving the appearance of the image.
The decision of whether or not to print a partial dot is made by comparing the pixels of the surrounding image to a set of templates. If a match is found, a correction is made to the center pixel. This is shown in the right side of FIG. 3. For example, the pixels within the template of the irregular shape shown at 81 is compared to the image surrounding the top dot of line 75. In this case there is a match, and the table lookup for this template will order that a dot of more than half but less than full intensity should be printed. Similarly, the image surrounding the bottom pixel will match template 87, and a pixel of less than half intensity will be produced. In a prior commercial printer, a large number of bits per template would be 49, and several hundred would be considered a large number of templates.
In a modern design facility, a software tool would be used to design the matrix of gates for the circuit for this matching process. Specifically, the number of inputs and outputs and each transfer function would be specified, and the tool would design a gate circuit that would perform the task.
Using this system, as the number and size of the templates rises linearly, the number of gates, and the time it takes for the resultant circuit to operate, rises at a compounded rate, and at sizes significantly larger than those given in the example above, the design of a commercially feasible circuit becomes impossible. What is needed is a circuit having a different configuration which can be used for a larger number of larger templates.